Inorg. Chem. 1988, 27, 47304738
4730
Figure 4. ORTEP diagram illustrating the hydrogen-bond arrangement between an inversion-relatedpair of [Fe(TPP)(OSO,H)] molecules.
of perchlorate metalloporphyrin derivatives,28another group of oxyanion-ligated species, are all close to Oo, an orientation that minimizes the nonbonded interactions between porphinato core atoms and the oxygens. In [Fe(TPP)(OS03H)],these interactions (28) (a) Spaulding, 1.D.; Eller, P. G.;Bertrand, J. A.; Felton, R. H. J . Am. Soc. 1974, 96,982-987. (b) Song, H.; Rath, N.; Reed, C. A,; Scheidt, W. R. TObe submitted for publication. (c) Barkigia. K. M.; Spaulding, 1 . D.; Fajer, J. Inorg. Chem. 1983, 22, 349-351. (d) Reed, C. A,; Mashiko, T.; Bentley, S. P.; Kastner, M. E.; Scheidt, W. R.; Spartalian, K.; Lang, G.J . Am. Chem. Soc. 1979,101,2948-2958. (e) Masuda.
H.; Taga, T.; Osaki, K.; Sugimoto, H.; Yoshida, Z.-I.;Ogoshi. H. Inorg. (0 Gans, P.; Buisson, G.;Duee, E.; Marchon, J.-c.;Erler, B. s.;Scholz. W. F.; Reed, c. A. J . Am. Chem. SOC. 1986, 108, 1223-1234. Chem. 1980. 19, 950-955.
are minimized by an increased value for the Fe-0-S bond angle (135.0°), an increase of 5-10° over the analogous angle in the various perchlorates. The unidentate HS04- ion in both molecules has the same interesting pattern in its S-0 distances. Each S-0 distance in the ion appears to be different from the others. As might be expected, the longest S-O distance in each ion involves the oxygen atom bonded to the hydrogen atom of the HS04- ion; the next longest involves the oxygen atom coordinated to the iron(II1) porphyrinate. The oxygen atom that is the hydrogen-bond acceptor is next longest, while the shortest distance involves the oxygen atom that is simply bonding to sulfur. For the reader's convenience, the S-0 distances of Table I11 are organized in the order of the above interactions; the strong similarity in values between the two molecules is evident. The hydrogen bonding between pairs of molecules in [Fe(TPP)(OSO,H)] and the resultant effect that leads to the various types of S-0 bonds (Table 111) suggest that there should be equivalent effects on the HS04- vibrational spectra. It is thus reasonable to suggest that the identity of the S02-I species is a non-hydrogen-bonded solid-state form of [Fe(TPP)(OSO,H)]. It further seems plausible that the observed hydrogen-bond interactions serve to stabilize the anion in its bisulfate form and retard the conversion of [Fe(TPP)(OS03H)] to [Fe(TPP)],SO,.
Acknowledgment. We thank the National Institutes of Health (Grant GM-38401-16) for support. Supplementary Material Available: Figure S1, showing an ORTEP diagram of molecule 2, and Tables SI-SV, listing complete crystal data and intensity collection parameters, anisotropic temperature factors for all atoms, fixed hydrogen atom coordinates, and complete tables of bond distances and angles in the two molecules (13 pages); a table of observed and calculated structure factors (XlO) (20 pages). Ordering information is given on any current masthead page.
Contribution from the NMR Section, Department of Radiology, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts 02 1 14
Solution Structure and Dynamics of Lanthanide(II1) Complexes of Diethylenetriaminepentaacetate: A Two-Dimensional NMR Analysis Bruce G . Jenkins and Randall B. Lauffer* Received June 27, 1988
The solution structure and dynamics of lanthanide(" complexes of diethylenetriaminepentaacetate(DTPA) have been investigated by 'HNMR. Two-dimensional (2D)exchange spectroscopy (EXSY) enables the determination of solution dynamics. At low temperatures (0-25 "C) the complexes Pr(DTPA)2-,Eu(DTPA)~-,and Yb(DTPA)*-are in slow exchange on the NMR time scale, and all 18 proton chemical shifts are resolved. Raising the temperature causes the number of resonances to decrease from 18 to 9, consistent with exchange between two enantiomers. The rates of exchange of the complexes follow the order Pr(II1) < Eu(1II) < Yb(II1). Utilization of 2D correlation spectroscopy (COSY) along with EXSY data as constraints enables assignments of the chemical shifts via calculations of the dipolar shifts. These calculations afford many reasonable solutions with low R factors, and only through the use of the 2D COSY and EXSY data can certain solutions be rejected. These calculations also confirm that the previously published crystal structure of Nd(DTPA)" can quite satisfactorily explain the solution structure. In addition, these data indicate that the central acetate moiety is coordinated to the metal in solution.
Introduction The solution structure and dynamics of multidentate lanthanide(II1) complexes are not well understood. These complexes generally have coordination numbers of 8-10 with greatly varying coordination Questions that arise include the following: (i) Are the solution structures the same as that in the solid state? (ii) Can the total coordination number vary as a function of temperature or across the lanthanide series? (iii) What (1)
(2)
Williams, R. J. P. Struct. Bonding 1982, 50, 79. Sinha, S. P. Strucr. Bonding 1976, 45, 69.
factors control the intramolecular dynamics? The importance of these issues has been enhanced by the recent development of gadolinium(II1) complexes as diagnostic pharmaceuticals for clinical 'H N M R imaging.3 These agents are injected intraveneously to alter the water proton relaxation times of tissues and improve image contrast and information content. The prototype agent, aquogadolinium(111) diethylenetriaminepentaacetate [Gd(DTPA)(H20)2-], has undergone extensive clinical trials4 and is now approved for human use in the U.S. It (3) Lauffer, R. B. Chem. Reu. 1987, 87, 901.
0020- 1669/88/ 1327-4730$01.50/0 0 1988 American Chemical Society
Lanthanide Complexes of DTPA is important to fully characterize solution chemistry of such chelates both to understand the in vivo behavior and to design improved agents. Critical features include the coordination of water to the metal center and the kinetic stability of the complexes in aqueous s ~ l u t i o n . ~ While transition-metal chelates of DTPA have been studied in solution,5s6 extensive studies of the lanthanide(II1)-DTPA chelates have not emerged. The crystal structures of the neodymium( 111) and gadolinium(II1) complexes reveal full octadentate chelation by DTPA and a single coordinated water.',* However, 'H N M R studies of the lanthanum(II1) and lutetium(111) derivatives have been interpreted as indicating that the central acetate is not coordinated in s o l ~ t i o n . ~This question is of importance with regard to the water proton relaxation efficiency of N M R contrast agents since an extra site for coordination of water to a paramagnetic metal has the potential of increasing the relaxivity considerably (cf. ref 3). In this study we utilize two-dimensional (2D) NMR techniques on both diamagnetic and paramagnetic Ln(DTPA)2- complexes to address the structural and dynamic aspects of these complexes in solution. These and our previous datal0 indicate that in solution Ln(DTPA)2- chelates exist in two symmetric forms that undergo dynamic chemical exchange. The rates of this exchange are comparable to those seen for the polyamino polycarboxylate ligands in Ln(D0TA)- and Ln(TETA)-;",'2 thus, Ln(DTPA)2- is relatively "rigid". In addition, our results provide unequivocal evidence for coordination of the central acetate.
Experimental Section DTPA Samples. Stoichiometric amounts of DTPA (Aldrich Chemical Co., 97% pure) and LnC13.6H20 (Aldrich 99.999% or Alfa 99.9%) were added to D 2 0 and the mixture stirred at 80 OC. Small amounts of concentrated NaOD were titrated into the solution until the pH was between 7 and 8 (uncorrected for deuterium isotope effect). The solutions were then filtered into N M R tubes to remove any residual solids. N M R spectra were obtained to ascertain the proper stoichiometry of the complexes. In some cases a small excess (C1:lO) of free DTPA was noted; however, no exchange with the Ln(DTPA)2- complexes was noted under any conditions studied. N M R spectra of La(DTPA)Z- were identical with those reported previo~sly.~ Partial deuteriation of the acetates was achieved by stirring a solution of free DTPA in D 2 0 at pH 11 (NaOD) at 90-95 OC. After 25 h under these conditions deuteriation of the terminal acetates was 54-57% and that of the central acetate was 44-45%. These ratios were determined by integration of the IH N M R spectra of the free ligands at pH 3. Deuteriation of the acetates was utilized to confirm assignments of proton resonances as belonging to either acetates or ethylenes. NMR Measurements and Calculations. N M R measurements were made at either 500, 300, or 250 MHz. Those at 500 MHz were collected at the Francis Bitter National Magnet Laboratory at the Massachusetts Institute of Technology on a home-built spectrometer. All other spectra were collected on either a Bruker AM300 or AM250 instrument at the Harvard University Chemistry Department. Chemical exchange was probed by using the standard 9O0-t1-9O0-~,-9O0 (NOESY) 2D pulse sequence" where tl is the evolution time during which spins develop frequency labeling and T,,, is the mixing time during which spins exchange magnetizations. Phase cycling was utilized to generate pure absorption mode spectra.I4 Since exchange effects are being monitored rather than nuclear Overhauser effects, we shall refer to these as EXSY spectra Runge, V. M., Claussen, C., Felix, R., James, A. E., Jr., Eds. Contrast Agents in Magnefic Resonance Imaging, Excerpta Medica: Princeton, NJ, 1986. Jezowska-Trzebiatowska,B.; Latos-Grazynski,L.; Kozlowski, H. Inorg. Chim. Acta 1977, 21, 145. Latos-Grazynski, L.; Jezowska-Trzebiatowska, B. J . Coord. Chem. 1980, 10, 159.
Stezowski, J. J.; Hoard, J. L. Isr. J . Chem. 1984, 24, 323. Gries, H.; Miklautz, H. Physiol. Chem. Phys. Med. N M R 1984, 16, 105.
Choppin, G. R.; Baisden, P. A,; Khan, S. A. Inorg. Chem. 1979, 18, 1330. Jenkins, B. G.; Lauffer, R. B. J . Magn. Reson. 1988, 80, 328. Desreux, J. F. Inorg. Chem. 1980, 19, 1319. Desreux, J. F.; Loncin, M . F. Inorg. Chem. 1986, 25, 69. Jeener, J.; Meier, B. H.; Bachmann, P.; Ernst, R. R. J . Chem. Phys. 1979, 71, 4546.
States, D. J.; Haberkorn, R. A,; Ruben, D. J. J . Magn. Reson. 1982, 48, 286.
Inorganic Chemistry, Vol. 27, NO. 26, 1988 4731 H2a b
Hlla,b
-0OC -CH,, -0OC - CH,
H9a b HlOa h
H 5 ~ bH 6 a b
N -CH,-CH,
'
-N -CH, I
CH-
- CH,
,CH,
- coo-
'CH,
- COO-
-N
C17n h
Figure 1. Schematic of the free ligand DTPA with proton labels corresponding to those in ref 7 and assigned to N M R chemical shifts for the Pr(II1) and Yb(II1) complexes in Tables I1 and 111. rather than NOESY spectra. Two-dimensional correlation spectra were acquired by using a phase-sensitive COSY.'s Chemical shifts are reported with respect to TSP. Some of the exchange rates reported were calculated from 2D EXSY spectra in the initial rate regime (Le. where T, 5 l/kex) by use of the following method. If we assume a simple two-site interchange between symmetry-related species (in which case k,,(forward) = k,,(backward)) where the protons have similar Tl's and intensities, then diagonal and cross-peak intensities are
I(cross) =
y2[1 - exp(-2k,,~,)]
[exp(-T,/
TI)]Zo
(1)
If 7, 5 l/kex (such that cross peaks have smaller intensities than diagonal peaks) then the above expressions can be rearranged to
Note that I, and Id represent volumes. Since the ratio of volumes is all that is necessary, we can determine the ratio of volumes by comparing the frusta of the approximate diagonal- and cross-peak cones: volume frustrum (cross) = y37rh,(R,2
+ r: + RJ,)
VOhme frUStrUm (diag) = %7rhd(Rd2 + rd2
Rdrd)
(3)
We then compare the cross- and diagonal-peak radii at two different contour levels (Le. R and r a r e the larger and smaller radii). In this case the heights, h , will be equal and hence
This method will work if we assume that apodization produces nearly circular line shapes and if contour levels are chosen intermediate between the base and vertex of the peaks. In addition, the only peaks chosen were those that were of equal intensity. The mixing times used in these k,, determinations varied between 0.5 and 1.5 ms. Other exchange rates were determined by coalescence temperatures from data obtained at both 500 and 300 M H z after correction for the temperature dependencies of the chemical shifts. Since the EXSY spectra allow us to assign all exchange partners, coalescence temperatures can be recorded over a wide temperature range, allowing for more accurate exchange activation parameters than are normally associated with this technique.16 This assumes, of course, that one can accurately correct for the temperature dependence of the paramagnetic shifts. The temperatures covered by this technique ranged from =O to 85 OC. Chemical shift calculations were performed by assuming both axial and nonaxial symmetry using the dipolar equations for the isotropic shifts (Le. G(iso) = G(LnDTPA) - G(LaDTPA))
3 cos2 0 - 1
sin2 e cos 2$
where r, 8, and 4 are the spherical polar coordinates of the proton relative to the magnetic susceptibility axes and D,and D2 are the dipolar constants that are related to the anisotropy of the magnetic susceptibility of the paramagnetic complex. The geometric factors above were determined from proton coordinates reported in the crystal structure of NdDTPA.' Fits to the observed chemical shifts were performed by using a computer program kindly provided by Dr. A. Dean Sherry,17 as well as a program (15) Bodenhausen, G.; Vold, R. L.; Vold, R. R. J . Magn. Reson. 1980, 37, 93. (16) Binsch, G. Top. Stereochem. 1968, 3, 97.
Jenkins and Lauffer
4732 Inorganic Chemistry, Vol. 27, No. 26, 1988
Table 111. Observed and Calculated Isotropic Shifts for Y b(DTPA)2-"
Table I. Kinetic Parameters for Conformational Dynamics of
LdDTPA)""
k,, Ln AG*,op AH' As 5 OC 8OoC Pr 56.5 f 3.6 35.2 f 2.0 -71.4 f 5.8 265 8,430 EU 55.4 i 4.6 38.5 f 2.4 -56.8 f 7.0 360 16,000 Yb 49.4 f 10.0 37.0 i 5.0 -41.7 i 16.2 4,300 163,000
" AG and AH values in kJ mol-'; AS values in J mol-'
K-'; k,, values
in s-l. Exchange rates reported from interpolations of Arrhenius plots except for k,, at 5 "C of Pr, which comes from EXSY data. Errors reported are f 2 standard deviations of fitted lines. Actual systematic
errors may be considerably larger. Table 11. Observed and Calculated Isotropic Shifts for Pr(DTPA)2-'
labelb r q P 0
n m 1 k
4 h
g f e d C
b
a
proton H9B H5B H9A H6A H6B H7B H7A H5A H2B HlOB HlOA H13B H2A H3B H13A HllB HllA H3A R
bob9
Ll,c
-45.3 -43.1
-43.2 -42.0
-19.7
-21.5 -21.4
-18.7 -12.7 -8.8
-8.6 -7.5
-2.0 +4.0 +4.7
+10.1 +14.5 +19.6 +27.3 +28.6 +36.0 +36.9
-12.7 -12.9 -10.9 -10.9 -8.0
+5.3 +1.2 +2.0 +10.0 +18.7
+19.8 +28.7 +36.2 +37.8 0.16
b C
d
e f g
h i j k 1
d
m n
-51.1
0
-47.4 -24.2 -24.0 -16.7
P q
L l C
r
proton H9B H5B H6A H9A H6B H5A H7B HlOA H7A HlOB H3B H13B H2A HllA H13A H11B H3A H2B R
+0.8
gob
LlcC
Llcd
+142.9 +109.5 +84.0 +7 1.8 +66.2 +41.6 +25.1 +16.4
+145.1 +102.7 +69.4 +78.8 +64.1 +44.0 +26.9 +17.7 +9.2 -1.9 -3.8 -8.6 -39.5 -49.4 -50.3 -52.5 -56.5 -64.1 0.094
+127.6 +95.7 +79.7 +77.0 +76.6 +40.0 +47.3 +18.1 +41.2 +17.0 -19.3 -6.4 -39.7 -38.2 -40.1 -45.5 -62.7 -52.4 0.232
+1.0 -1.1
-8.1 -12.5 -35.9 -38.0 -50.7 -5 1.2 -53.5 -66.2
'Shifts are reported for the best fit to data calculated from proton coordinates of the crystal structure of NdDTPA,' with Nd representing the origin of the coordinate system. The best-fit axes correspond to 0 = 19.6O, 6 = 5 7 . 2 O for the neodymium-water oxygen vector. bLabels correspond to the chemical shift labels shown in Figure 2c. 'Calculated by using eq 6 in text, with Di = 3880 f 99 ppm A3 and D2 = 3397 i 307 ppm A'. dCalculated by using eq 5 in text, with D, = 3957 f 229 ppm A'.
-5.5 -17.2
+3.3 +1.3 -6.1 -5.3
+3.4 +7.3 +11.0 +16.1 +23.8 +22.0 0.40
In order to probe this exchange process in more detail, 2D EXSY spectra were utilized. This technique allows one to sample
'Shifts are reported for the best fit to data calculated from proton coordinates of the crystal structure of NdDTPA.' Nd represents the origin of the coordinate system with the best-fit axes corresponding to the neodymium-water oxygen vector (see Figure 4) making an angle of 16.0' with the z axis and the projection of that vector in the x , y plane making an angle of 10.Oo with the x axis (Le. 0 = 16.0°, #J = 10.0' for that vector). bProton labels correspond to those in Figure 2a. CCalculatedby using eq 5 in text, with D I = -1250 i 125 ppm A3 and D2= -1250 i 300 ppm A'. dCalculated by using eq 6 in text, with D, = 1430 i 25 ppm A'. written by us utilizing a different iterative method (unpublished). Both methods minimize the agreement factori8 (7)
Results and Discussion Solution Dynamics of Ln(DTPA)2- Complexes. Shown in Figure 1 is the structure of DTPA with proton labels assigned from the crystal structure of NdDTPA.7 The letters correspond to proton assignments reported later in Tables I1 and 111. One-dimensional IH NMR spectra as a function of temperature are shown in Figure 2 for Pr(DTPA)Z-, Eu(DTPA)'-, and Yb(DTPA)2-. At low temperatures 18 resonances are visible for Pr(DTPA)2-, though the peaks marked 1 and m overlap considerably. At low temperatures 18 resonances can also be seen for Eu( DTPA)", though one signal is buried beneath the HOD signal, and two at ca. -15 ppm overlap above 2 O C . For Yb(DTPA)2two proton resonances, one acetate resonance and one ethylene resonance are obscured by solvent signals; however, none overlap. At high temperatures there are nine resonances for all three chelates, indicating that an exchange occurs between two symmetry related species. Sherry, A. D.; Tehrani, J. J . Biol. Chem. 1983, 258, 8663. (18) Hamilton, W. R. Acra Crystallogr. 1965, 18, 502. Willcott, M. R., 111; Lenkinski, R. E.; Davis, R. E. J . Am. Chem. SOC.1972, 94, 1742. (17)
labelb a
the entire exchange matrix in one experiment rather than perform a large number of saturation transfer experiments. The main problem with utilization of a technique involving incoherent magnetization transfer (such as EXSY) with paramagnetic molecules is shortened Tl's.Io If l/T1 is much greater than k,,, then transverse magnetization will be destroyed by spin-lattice relaxation before appreciable cross-peak intensity can accumulate. By suitable adjustment of the mixing time (+,,,) this problem can be alleviated to a degree. Shown in Figure 3 are EXSY spectra for Pr(DTPA)2-, Eu(DTPA)2-, and Yb(DTPA)2-. A complete assignment of the exchanging partners is easily obtained by tracing the proton exchange connectivities. The spectra for Eu(DTPA)~-and Pr(DTPA)2- are extremely clean, whereas that of Yb(DTPA)2contains considerably more noise. The relaxation times are much shorter in the case of Yb(DTPA)2-, and this leads to a much more rapid decay of signal intensity.1° Nonetheless, all the exchange pairs are easily resolved. From the simple coalescence behavior (18 peaks to 9), we deduce that exchange is occurring between two enantiomers of each of the three complexes (i.e,, racemization). The crystal structure of Nd(DTPA)2- indeed reveals a chiral complex that is viewed most straightforwardly as a monocapped square antiprism.7 Three nitrogens and one oxygen from a terminal acetate form the bottom square face and the four remaining acetate oxygens form the top face; the 9-coordinate complex is capped with a water molecule (Figure 4). The major feature of the exchange process essentially involves the shuffling of coordinated acetates as shown in Figure 4: the lone terminal acetate in the bottom face is displaced to the top face while another terminal acetate takes its place. The positions of the other acetates (and possibly the water molecule) are also altered in this process. In addition, the ethylenes flip between staggered conformations. (Evidence for solution structures similar to the Nd(II1) crystal structure is presented below.) Exchange rates for this racemization process can be calculated from coalescence temperatures. Since there are so many protons, we can follow the 1D spectra as a function of temperature and obtain a number of different coalescence temperatures. In addition, following the coalescence at two (or more) frequencies
Inorganic Chemistry, Vol. 27, No. 26, I988 4733
Lanthanide Complexes of DTPA
'
-I?
-? -? -? -0
-R a-=
